docs: def Kp_helical_ribbon_Rieger(D, h, nb, pitch, width, T):
      r'''Calculates product of power number and Reynolds number for a
      specified geometry for a heilical ribbon mixer in the laminar regime.
      One of several correlations listed in [1]_, it used more data than other
      listed correlations and was recommended.
  
      .. math::
          K_p = 82.8\frac{h}{D}\left(\frac{c}{D}\right)^{-0.38} \left(\frac{p}{D}\right)^{-0.35}
          \left(\frac{w}{D}\right)^{0.20} n_b^{0.78}
  
      Parameters
      ----------
      D : float
          Impeller diameter [m]
      h : float
          Ribbon mixer height, [m]
      nb : float:
          Number of blades, [-]
      pitch : float
           Height of one turn around a helix [m]
      width : float
           Width of one blade [m]
      T : float
          Tank diameter, [m]
  
      Returns
      -------
      Kp : float
          Product of Power number and Reynolds number for laminar regime []
  
      Notes
      -----
      Example is from example 9-6 in [1]_. Confirmed.
  
      Examples
      --------
      >>> Kp_helical_ribbon_Rieger(D=1.9, h=1.9, nb=2, pitch=1.9, width=.19, T=2)
      357.39749163259256
  
      References
      ----------
      .. [1] Paul, Edward L, Victor A Atiemo-Obeng, and Suzanne M Kresta.
         Handbook of Industrial Mixing: Science and Practice.
         Hoboken, N.J.: Wiley-Interscience, 2004.
      .. [2] Rieger, F., V. Novak, and D. Havelkov (1988). The influence of the
         geometrical shape on the power requirements of ribbon impellers,
         Int. Chem. Eng., 28, 376-383.
      '''
      c = 0.5*(T - D)
      return 82.8*h/D*(c/D)**-.38*(pitch/D)**-0.35*(width/D)**0.2*nb**0.78